(1) Field of the Invention
This invention relates generally to hydrophone modules that are towed by ocean vessels. More particularly, the present invention relates generally to hydrophone units that are mounted between bulkheads of modular arrays towed by surface ships and submarines.
(2) Description of the Prior Art
Arrays of hydrophones in modules are conventionally towed by surface ships and submarines for the purpose of sensing sound below the surface of the ocean. Typically, such arrays are linear assemblies of coupled modules, each module comprised of hose sections that have bulkheads at opposing ends. Hydrophones are mounted in the hose sections. The hose sections are filled with fluid that surrounds the hydrophones. Sound pressure waves in the ocean pass through the hose wall and into the fluid that surrounds the hydrophones. The hydrophones sense the pressure fluctuations and transform the sensed pressures into electrical signals which are transmitted via a cable back to the vessel. The electrical signals are then processed to derive the sound or a representation thereof.
During the towing of the array, the hose walls of the linear array and the center strength member, if the array has one, may be subject to extensional waves transmitted through the tow cable. The extensional waves may have a motion component produced by differential motion of the vessel and also a vibrational component produced by the forces exerted on the cable due to vortex shedding. Because the bulkheads disposed at the ends of the modules are mechanically connected to the hose wall, the bulkheads are driven into an oscillatory state by the extensional waves propagated within the hose wall.
When the bulkheads oscillate, their motion generates pressure waves in the fluid, on each side of each bulkhead. The pressure waves are sensed by adjacent hydrophones and transmitted electronically as noise. The hose wall responds to the pressure waves by alternately expanding (or bulging) and contracting as the waves travel through the fluid, thus producing a "breathing" effect in the system. As the hose wall expands, its circumferential stiffness determines the amplitude of the pressure waves. These "breathing waves" introduce a significant noise factor into the sound sensing process.
It is well established that the amplitude of these pressure waves is a function of the effective stiffness of the system comprised of the hose wall filled with fluid. Given two hoses filled with fluid of the same density, the effective stiffness of the system is determined by the circumferential stiffness of the hose wall. Various modifications have been made to the hose wall in attempts to decrease the energy produced by the breathing waves. Nevertheless, significant quantities of system noise from this source are received by the hydrophones, and at this time even sophisticated electronic processing cannot remove the noise.